Recent research into meta-materials has enabled microscopic control and macroscopic control for an electromagnetic field (see Phys. Rev. Lett. 85, 3966 (2000); Science 312, 1777 (2006); Science 312, 1780 (2006)). A meta-material is a material in which an electromagnetic characteristic that cannot be realized in a general natural state is realized using an artificial method. A meta-material is characterized in that it has a negative refractive index, and thus light is bent in the direction, opposite to a direction in which the light is bent in a normal material, in the meta-material.
A scheme for freely adjusting the direction of an electromagnetic field regardless of the source of the electromagnetic field and also providing guidance while avoiding an object as if there was no object by using such a meta-material was proposed (see Science 312, 1777 (2006); Science 312, 1780 (2006)). This scheme can be potentially applied to radiation shielding from a strong electromagnetic pulse (EMP) or electromagnetic energy having directionality.
Electromagnetic field control using a meta-material is attracting increasing attention in the fields of novel applications, such as an invisibility cloak, a concentrator, and a refractor.
Among these applications, an invisibility cloak is intended to hide an object inside a given geometrical shape, and is the most attractive application. An invisibility cloak is based on the coordinate transformation and conformal mapping of Maxwell's equations, and such invisibility cloaks were independently proposed by Pentry (see Science 312, 1780 (2006)) and Leonhardt (see Science 312, 1777 (2006)).
A full wave electromagnetic simulation of a cylindrical cloak using ideal or non-ideal electromagnetic parameters has been researched, and the experimental implementation of a cylindrical cloak having simple parameters, which operates at a microwave frequency, was announced.
In the analysis and design of an invisibility device, it is most important to calculate permittivity and permeability tensors for a meta-material that constitutes a cloaking shell.
It is assumed that an invisibility device distorts field lines so that the field lines move while avoiding any area having uniform field lines in the corresponding area. This distortion may be considered to be coordinate transformation between an original Cartesian mesh and a distortion mesh.
The theory and experimental implementation of the conventional invisibility device is significantly influenced by the propagation direction of an electromagnetic wave, polarized light, and a wavelength band. Although a technology for improving the efficiency of an invisibility device by using complementary media was proposed in the paper “Complementary media invisibility cloak that cloaks objects at a distance outside the cloaking shell,” Y. Lai, H. Chen, Z. Q. Zhang, and C. Chan, Phys. Rev. Lett. 102, 93901 (2009) (published on May 2, 2009), this preceding technology self-proclaims that it is valid only at finite frequencies.
Attempts to overcome this limitation and extend the preceding technology to a theory that is applicable to more general cases were introduced in the paper “Calculation of Permittivity Tensors for Invisibility Devices by Effective Media Approach in General Relativity”, Doyeol Ahn, Journal of Modern Optics, Volume 58, Issue 8, 2011 (published on Apr. 4, 2011) and Korean Patent Application Publication No. 10-2013-0047860 (published on May 9, 2013).
In the approaches of the preceding technologies, permittivity and permeability tensors may be scaled using factors obtained via coordinate transformation or optical conformal mapping technology while maintaining the forms of Maxwell's equations that do not change in any coordinate system.
Furthermore, a method for calculating permittivity and permeability tensors for an invisibility device by using electrodynamics in the frame of the theory of relativity was researched.
The principle idea of this preceding technology is based on the fact that in curved space-time, the propagation of an electromagnetic wave appears as wave travelling in an inhomogeneous effective bi-anisotropic media. The constitutive parameters thereof are determined by a space-time metric.
This technology can express the inverse problem of transformation into any curved space-time in a media inside flat space-time, and can find specific conditions for invisibility cloaking.
The above-described preceding technologies relate to invisibility techniques in which a cloaking target is limited to an electromagnetic wave. There is no embodied preceding technology in which a cloaking target is an acoustic wave.